--- title: "ASK and FSK modulators/demodulators; PSK with envelope detector" subject: "Communication Systems" chapter: 7 key: "semester_6/communication_systems:ask-fsk-mod-demod-psk-envelope" source: https://www.bctengineer.com/q/semester_6/communication_systems/ask-fsk-mod-demod-psk-envelope site: BCT Engineering --- # ASK and FSK modulators/demodulators; PSK with envelope detector Communication Systems · Chapter 7: Digital Modulation Techniques - Asked in: 2065 Kartik (CS I), 6+2 marks - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/communication_systems/ask-fsk-mod-demod-psk-envelope - Whole chapter: https://www.bctengineer.com/md/semester_6/communication_systems/chapter-7 ## Question Explain ASK and FSK modulators and demodulators with the help of block diagrams. Can PSK wave be detected using envelope detector? Explain. ## Short answer ASK switches carrier amplitude on/off; FSK switches between two frequencies. Both can be demodulated coherently or with envelope detectors. PSK cannot be envelope-detected because its amplitude is constant for both phases, so the envelope detector output is identical and carries no information; coherent detection or DPSK is necessary. ## Answer #### ASK modulator In **Amplitude Shift Keying (on-off keying)** the carrier is sent for bit 1 and switched off for bit 0: $s(t)=b(t)\,A\cos 2\pi f_ct$, with $b(t)\in\{1,0\}$. ```text binary (unipolar NRZ) b(t) ---->(X)----> ASK ^ A cos 2 pi fc t (oscillator) ``` The product modulator (or simply a switch) passes the carrier only during 1s. #### ASK demodulators Coherent: ```text ASK -->(X)-->[integrate 0..Tb]-->[compare with Vth]--> data ^ cos wc t (carrier recovery) ``` Non-coherent (envelope detection): ```text ASK ->[BPF at fc]->[envelope det]->[comparator]-> data ``` The output is high when the carrier is present (1) and near zero when it is absent (0); the threshold is about half the peak. #### FSK modulator In **Frequency Shift Keying** bit 1 is sent at $f_1$ and bit 0 at $f_2$. ```text b(t) --------------->(X)<-- cos w1 t --+ | (+)--> FSK +-->[Inverter]---->(X)<-- cos w2 t --+ ``` Equivalently, the polar NRZ data can drive a VCO, whose output frequency switches between $f_1$ and $f_2$ (giving continuous phase). #### FSK demodulators Coherent: ```text +->(X)<-cos w1 t ->[int 0..Tb]--+ FSK ---+ (-)--> l --> l>0: 1 +->(X)<-cos w2 t ->[int 0..Tb]--+ l<0: 0 ``` Non-coherent: ```text +->[BPF f1]->[envelope det]--+ FSK ---+ [compare]--> data +->[BPF f2]->[envelope det]--+ ``` The branch with the larger envelope decides the bit. A PLL demodulator can also be used. #### Can PSK be detected with an envelope detector? **No.** In binary PSK, $s(t)=\pm A\cos 2\pi f_ct$. The amplitude is $A$ for both 1 and 0; only the phase differs by $180^\circ$: $$ \text{envelope of } +A\cos\omega_ct = \text{envelope of } -A\cos\omega_ct = A $$ An envelope detector responds only to amplitude, so its output is a constant $A$ for every bit and all information is lost. PSK must be detected coherently (multiply by a locally recovered carrier and integrate, the sign gives the bit), or in differential form (DPSK) by comparing each bit with the previous one. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. How is OOK ASK generated? A. Polar data multiplies the carrier B. Unipolar NRZ data multiplies the carrier, passing it only for 1s C. Data drives a VCO D. Two oscillators are switched by data Answer: B. s(t) = b(t) A cos(2 pi fc t), with b in {1, 0}. (Key point: ASK modulator) 2. How does the non-coherent FSK demodulator decide a bit? A. A single BPF and a zero-threshold comparator B. Multiplication by recovered carriers and a sign decision C. BPFs at f1 and f2 feed envelope detectors, and the larger envelope wins D. Squaring the signal and dividing the frequency Answer: C. Compare the tone strengths at f1 and f2. (Key point: FSK non-coherent) 3. How does a VCO produce FSK? A. It multiplies the data by a fixed carrier B. It adds two oscillator outputs C. It inverts the phase for bit 0 D. Polar NRZ data switches its output frequency between f1 and f2 with continuous phase Answer: D. Frequency follows the data level, giving continuous-phase FSK. (Key point: VCO FSK) 4. Can an envelope detector detect BPSK, and why? A. No, because the envelope is A for both +Acos and -Acos B. Yes, because phase flips change the envelope C. Yes, if the threshold is set to half the peak D. No, because PSK has a lower power than ASK Answer: A. Only the phase differs by 180 degrees; amplitude is the same, so the detector output is constant. (Key point: PSK and envelope detector) ## Related in Communication Systems - [Need for shift keying; ASK vs FSK vs PSK](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Ashift-keying-ask-fsk-psk) — Communication Systems, chapter 7 - [Coherent ASK and PSK error; ASK needs double power](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Aask-psk-error-double-power) — Communication Systems, chapter 9 - [Envelope detector; can it demodulate DSB-SC?](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Aenvelope-detector-dsbsc) — Communication Systems, chapter 2, asked 2× - [Square law detector limitation; envelope detector](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Asquare-law-detector-limitation-envelope) — Communication Systems, chapter 2 - [Envelope detector circuit and waveforms](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Aenvelope-detector-circuit) — Communication Systems, chapter 2 - [Envelope detector; why not for DSB-SC and SSB](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Aenvelope-detector-why-not-dsbsc-ssb) — Communication Systems, chapter 2 ## Related in other subjects - [ASK, FSK and PSK modulation](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Aask-fsk-psk) — Data Communication, chapter 5, asked 3× - [FDM and TDM with block diagrams](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Amultiplexing-fdm-tdm-block) — Telecommunications, chapter 3 - [Space diversity techniques with block diagrams](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Aspace-diversity-block-diagrams) — Wireless Communication, chapter 5 - [Standard and ladder programming block diagrams](https://www.bctengineer.com/question.md?key=semester_6%2Fdigital_control_system%3Afilter-standard-ladder-block) — Digital Control System, chapter 4